Solid state image sensors fabricated from semiconductor materials are well known. A popular class of such device is produced using CMOS processing technologies, which again are well known. An image sensor comprises an image sensing array formed by a number of pixels, and associated circuitry for controlling the operation of the image sensing array and for manipulation of the signals that are output from the image sensing array.
Within the image sensing array, each pixel comprises a photosensitive portion formed for example as a doped region within a semiconductor substrate. The energy of incident photons removes electrons from the outer orbits of atoms within the photosensitive portion thus generating a charge. The photosensitive portion can take different forms, for example a photodiode, photogate, phototransistor, charge-coupled device (CCD), charge injection device (CID), or single photon avalanche diode (SPAD), among others.
The pixel also comprises readout circuitry. There are many different architectures for pixels, including for example the so-called 3T and 4T (pinned photodiode) structures. Other structures such as a so-called 2.5T and a 1.75T pixel are known, which uses shared read-out lines between successive rows of pixels to effectively reduce the number of transistors required per pixel. In any event, regardless of its architecture, the readout circuitry of a pixel will in general provide switching circuit or means for the connection of the photosensitive portion to an output node connected to a readout bus for the transmittal of the charge as a voltage signal, and a reset node for connection with a reset voltage source, and optionally further reference voltages and/or control lines as appropriate. Further components can be incorporated in the pixel's readout circuitry as a design choice made for particular applications. For example, analog-to-digital converter (ADC) circuitry for some applications could be incorporated in the image sensor's associated circuitry, i.e. separate from the image sensing array. However as an alternative, each pixel can be provided with its own ADC circuitry. As another example, an amplifier can be incorporated separate from the image sensing array, or it can be distributed, with one or more of its components being formed as part of the pixel circuitry.
No matter the design choices made in an image sensor's architecture, the term “pixel” is used to refer to a unit element of an image sensing array for generating information about a scene. The photosensitive portion of a pixel makes use of an electric field at a P-N junction to cause the photogenerated electron to move away from the ion and prevent re-combination and loss of the signal. However, these P-N junctions have a small leakage current which the photosensitive portion cannot distinguish from a current which is generated by light. This leakage current is present in the dark, and so this leakage current is commonly known as dark current. The term “dark” is understood to be a condition where light is either absent, or where light that is incident on the image sensor does not cause the photogeneration of charge by the pixels' photosensitive portions. This can either be because the photosensitive portions are shielded, or because they are held at a potential, for example a reset potential, that prevents the accumulation of charge at the photosensitive portions.
This dark current is a primary limiting factor in the performance characteristics of photosensitive portions in CMOS image sensors. Dark current is strongly temperature dependent and this makes it difficult to compensate for. It also varies considerably with any non-uniformity in doping gradients across the die or non-uniformity introduced by etching variations.
One technique known for compensating for dark current is disclosed in European Patent Application EP 1544602A to STMicroelectronics Limited. This application discloses a method wherein a selection of pixels within an array are shielded from incident light and their output is compared with the output of the remaining pixels in an attempt to cancel the dark current. However, the variation of dark current that occurs between successive pixels or successive regions of pixels may still cause an error with a system of this nature. Furthermore, because the signals are taken from the outputs of different pixels further errors are introduced in the switching and transfer of charge needed to obtain the outputs and then compare them.
It is also known to use a shielded border portion of a pixel array to generate a dark current reading which can be subtracted from the outputs of the image sensing array. However, this does not take into account the variation of dark current across the die.
Another technique for cancelling or compensating for dark current noise is to take two reads as disclosed for example in U.S. Pat. No. 4,942,474; U.S. Pat. No. 6,067,113, U.S. Pat. No. 5,376,966 and U.S. Pat. No. 5,642,162. These techniques involve the use of a mechanical shutter to enable or prevent light from reaching the sensor. A first reading is taken without light but with the dark current and a second reading is taken with the light and also the dark current. The two readings are then differenced in an attempt to cancel the dark current. However, as dark current varies with time, the time difference between the successive readings can introduce an error. Furthermore, the action of the first read causes a “self heating” effect, as the power consumed in order to produce the first reading actually increases the temperature of the device and so alters the dark current prior to the second reading. Thus there is an inherent error in the dark current cancellation in these techniques. Furthermore, the use of a mechanical shutter is not suitable for small, lower power schemes.
These problems of spatial and temporal variation of dark current are felt keenly in applications that require the detection of a very small amount of light. Various scientific and industrial applications fall into this category, requiring for example to detect light levels of only a few 10,000 photons per second. One example of a image sensor in this category is a bioluminescence sensor in which an analyte is passed over a reagent provided at the sensor. The chemical reaction between the analyte and reagent produces light, which is then measured and analyzed to provide details about the nature of the analyte. These types of sensors are popular as they do not require a stimulating light source. However, the photo emissions are small. Existing systems use a separate detection system often with separate photo-multiplier systems which add expense and complicate integration (readout) of signal.
Accordingly, there is a need for an improved system and method of dark current cancellation. Such improvements would be useful in the field of image sensors in general, but would be particularly appreciated in fields where an image sensor is used to detect relatively low light levels, such as bio-luminescence sensors, where the light levels of radiation incident on the image sensing array are much smaller than for example the light levels that would be incident on an image sensing array of a digital still camera or mobile telephone camera used in normal domestic circumstances.